Recently, the need for a toner capable of ensuring fixability at low temperature and storage stability at high temperature has increased in the printing industry.
Generally, a toner is prepared by adding a colorant, a charge control agent, a releasing agent, or the like to a thermoplastic resin acting as a binder resin. In addition, inorganic fine metal particles such as silica or a titanium oxide may be added to a toner as external additives in order to provide a toner with fluidity or improve its physical properties such as charge controlling properties or cleaning properties.
The methods of preparing such a toner can be categorized into physical and chemical methods.
The physical methods include a pulverization method. The pulverization method is a method of preparing a toner by obtaining a toner composition by melt-mixing a colorant, a charge control agent, and the like, with a binder resin, such as a polyester resin, dispersing the melt-mixture homogenously, and then pulverizing and classifying the toner composition. However, the pulverization method requires a pulverizer and related equipment for pulverizing the toner composition and thus high manufacturing costs are incurred in order to prepare small particulate toner particles. It is easy to generate toner particles having a wide particle size distribution during the pulverizing of the toner composition. In addition, there is a need to classify and remove fine toner particles having a particle size equal to or less than 3 μm and large toner particles having a particle size equal to or greater than 20 μm in order to obtain an image having high resolution and high gradation. Also, when an additive is not uniformly dispersed, the fluidity, developing properties, durability, and image quality of the toner may be adversely affected.
The chemical methods include a suspension-polymerization method and an emulsion-aggregation method.
The suspension-polymerization method is a method of preparing a toner by suspension-polymerizing toner materials in a solvent. The patent rights for such kinds of technologies are generally owned by firms such as Canon or the like. (U.S. Pat. No. 6,177,223). Problems of the above-mentioned pulverization method can be overcome by the suspension-polymerization method. However, in the suspension-polymerization method, since only styrene-acrylic copolymers can be used as a basic resin and toner particles prepared thereby are spherical in shape, cleaning properties may be poor. Thus, a toner may remain on a photosensitive medium, and thus quality of images formed thereby may deteriorate. As such, contamination will occur on a charging roller, etc. and charging properties may be adversely affected.
The emulsion-aggregation method is an example of a chemical method of preparing a toner composition (U.S. Pat. Nos. 5,916,725 and 6,268,103). The emulsion-aggregation method involves preparing a micro-emulsion resin particle composition through an emulsion polymerization reaction and then aggregating the composition with other toner components, e.g., a pigment dispersion and a releasing agent dispersion. In the emulsion-aggregation method, the above problems of the pulverization method can be overcome, and toner particles may have non-spherical shapes by adjusting aggregating conditions. However, only styrene-acrylic copolymers can be used as a binder resin, and preparation of a separate dispersion needs to be further included.
In such chemical methods, only styrene-acrylic copolymers can be used as a binder resin. Thus, polyester resins, which have excellent fluidity, and improved pigment dispersibility and transparency due to their own chemical structures, generally cannot be used in toners for a color printer and a high-speed printer.
An example of a method of preparing a toner using a polyester resin is a method using a self water dispersible polyester resin (U.S. Pat. No. 5,916,725). However, a sodium sulfonate group needs to be included in a resin for self dispersion in water. In addition, a toner including many such functional groups included in the resins may have poor stability since the toner is likely to be affected by external environmental factors such as moisture, etc. which are encountered after manufacture.
JP Patent Nos. 3,640,918, 3,895,172 and 3,878,537 disclose a dry toner including a toner binder (i.e., a binder resin) and a colorant. In this case, only a modified polyester resin is used as the toner binder, and preferably, the modified polyester resin together with an unmodified polyester resin is used as the toner binder. An example of the modified polyester resin may include a polyester prepolymer including an isocyanate group, or the like. Examples of the modified polyester resin may include a polycondensate of polyol and poly carboxylic acid, or the like. However, such preparation methods are complicated in that the prepolymer is prepared by way of an additional reaction in which an isocyanate is included in oligomer and the prepolymer is reacted with a cross-linking agent and/or an elongation agent so as to prepare a toner particle.